1. Field of the Invention
The present invention relates to a wire electric discharge machine controller for correcting the machining route using program commands.
2. Description of the Related Art
In wire electric discharge machining, a problem is known that the accuracy of corner shapes of a workpiece is degraded by the deflection of a wire electrode that occurs due to the flow of machining liquid and/or a discharge repulsion force arising between the wire electrode and the workpiece. FIG. 5 shows wire electric discharge machining at a corner. Being deflected, the wire electrode 4 advances along the track 8 indicated by the dashed line inside the machining route 6 that is created on the basis of a machining program. Consequently, the workpiece 2 would be machined more than expected, resulting in degraded accuracy of the shape. Reference numeral 10 in FIG. 5 indicates the portion of the workpiece excessively machined by the deflected wire electrode.
Wire electric discharge machining methods and controllers have been designed to improve the machining accuracy by controlling the machining speed and/or energy at corners or by correcting or changing the machining route at corners. Since controlling the machining speed and/or energy to secure the shape accuracy at corners involves adverse effects such as a longer machining time, various methods have been designed to correct or change the machining route without significantly affecting the machining time.
Generally contemplated first would be a method that changes the machining route at corners by modifying the machining program to cause the wire electrode to advance outside the original machining route as shown in FIG. 6. Reference numeral 12 in FIG. 6 indicates the changed machining route. When machining a shape as indicated by the solid line in FIG. 7, the worker modifies the axis movement commands corresponding to the machining blocks in the machining program as shown in FIG. 8 to change the machining route at the corners as indicated by the dashed lines.
In such a method, the worker creates a machining route using movement commands in the machining program by taking into account the amount of deflection of the wire electrode, such that the deflection of the wire electrode does not affect the workpiece. For a simple shape as in FIG. 7, it is easy to modify the machining program as shown in FIG. 8, but not easy when the shape to be machined is complicated. Suppose modification of a machining program for machining a shape requiring simultaneous X and Y axis operations as shown in FIG. 9, for example. Assuming that the coordinates of point B are (Xb, Yb) and the coordinates of point C are (Xc, Yc), then the coordinates of point B′ become (Xb+L1, Yb) and the coordinates of point B″ become (Xb−L2×cos θ, Yb−L2×sin θ) and accordingly the amounts of X and Y axis movements between these points are expressed as follows:                Section BB′: (L1, 0)        Section B′B″: (−L1−L2×cos θ, −L2×sin θ)        Section B″C: (Xc−Xb+L2×cos θ, Yc−Yb+L2×sin θ)        
It is not easy to modify the machining program such that these coordinates are passed through. Accordingly, a technique such as the following has been designed to automatically correct the machining route, eliminating the need for the worker to directly modify the machining program. This technique automatically identifies corners and their angles etc. from the machining program and automatically corrects the machining route by retrieving the amounts of correction from the parameters stored in a storage device or the like, thereby eliminating the need for the worker to modify the machining program.
Japanese Patent Application Laid-Open No. 61-219529 (hereinafter referred to as Patent Document 1) discloses a shape control system of wire electric discharge machining. This control system includes a storage unit for storing the amount of deflection of the wire electrode, a computing unit for determining the machining direction of the wire electrode by computation based on the shape to be machined, and a drive unit for driving the wire electrode after positioning it forward in the machining direction by the amount of correction that is equal to the amount of deflection.
Japanese Patent Application Laid-Open No. 7-24645 (hereinafter referred to as Patent Document 2) discloses a wire electric discharge machining for correcting a machining route when a corner is detected in the machining route, such that the wire electrode moves on a machining route extended by a specified distance in the advancing direction, then moves along the original machining route for a specified distance, and then gradually returns to the original machining route.
Since there are a great variety of machining types performed by a wire electric discharge machine, a great amount of experiments and verifications are required to set up various parameters to appropriately control these types of machining. Even if the sizes or angles of the corners are identical, appropriate machining could not be achieved if the amounts of correction are not changed depending on the machining environment such as the presence or absence of disturbances or other influences. The prior art methods, therefore, cannot always achieve machining results as intended by the worker.
In the technique disclosed by the aforementioned Patent Document 1, the amounts of deflection of wire electrode are stored in the storage unit and the machining route is corrected by positioning the wire electrode using these amounts of deflection as the amounts of correction, thereby relieving the worker of the aforementioned burden of modifying the machining program. This technique, however, has several problems yet to be solved.
One of the problems is that the worker cannot set the amounts of correction as desired because the amounts of correction are automatically determined from the amounts of deflection of the wire electrode stored in the storage unit. In wire electric discharge machines, electrical machining conditions are determined according to the thickness and material of the workpiece, as well as the thickness and material of the wire electrode, and numerous machining conditions are stored in the controller. To determine the amounts of deflection of the wire electrode, the influences of the machining conditions must be taken into account. At the same time, disturbances and influences from the machining environment cannot be ignored, including changes in flow of the machining liquid due to differences in shape of the corners, i.e., inside corners or outside corners, the angles of the corners, the change in thickness of the workpiece, and interference with a jig.
In Patent Document 1, when the amount of deflection under predetermined machining conditions has not been stored, the corresponding amount of deflection is determined by performing test machining according to a control program. In this way, the amount of deflection can be set by taking into account the influences of machining conditions. However, since the aforementioned disturbances and influences from the machining environment are not taken into account, machining cannot always be achieved as intended by the worker. In other words, this technique cannot address the influences and the amounts of correction cannot be changed when the machining environment varies even if the sizes and angles of the corners are identical.
The technique disclosed by Patent Document 2 is schematically shown in FIG. 10. As shown in FIG. 10, a machining program is stored in a machining program storage unit 20. A corner detecting unit 21 detects corners in the machining routes identified from the machining program. When the corner detecting unit 21 detects a corner in the machining route, a machining route correcting unit 22 corrects the machining route at the corner such that the wire electrode deviates from the original machining route at the corner by a predetermined distance in the advancing direction, then moves along the next machining route for a predetermined distance from the corner, and gradually returns to the original machining route, and outputs the machining route thus corrected to a machining route controlling unit. On the basis of the machining route received from the machining route correcting unit 22, the machining route controlling unit 23 moves the wire electrode with respect to the workpiece.
As shown in FIG. 10, the technique disclosed by Patent Document 2 detects corners in the machining route using a corner detecting unit 21 and automatically corrects the machining route by predetermined distances set in parameters. These parameters are set by distinguishing inside corner from outside corner and/or on the basis of the angles of the corners, etc. Since a great amount of time and effort is required to prepare various patterns of parameters for different machining conditions, several typical parameters are prepared in practice and the parameters seemed closest to the machining situation are selected. Consequently, machining cannot always be achieved as intended by the worker.